A US study has found no evidence that vaccination for mumps, measles and rubella is associated with an increased risk of autism.

Recent reports in Britain show the risk of autism has jumped sharply in the past decade, and some researchers have argued that autism may be triggered by vaccination, introduced in 1988.

The study, published this week in the British Medical Journal by researchers at the Boston University School of Medicine, did indeed find that autism had increased fourfold in boys aged between two and five years old who were born between 1988 and 1993. However, the authors say they found no association between these boys and the measles, mumps and rubella (MMR) vaccine.

To determine if there was an effect, the authors set out to compare autism in children who had been vaccinated versus those who had not. However, only about three per cent of children did not receive MMR, making the sample size too small for comparison.

Instead, they analysed information from 114 boys born between 1988 and 1993 who were diagnosed with autism between the ages of two and five.

Although the risk of being diagnosed with autism did jump four-fold during this period, they found vaccination levels didn't change during the period when autism rates were rising.

In addition, children are usually vaccinated at 15 months old, while autism is not typically diagnosed until after the age of two.

So why, then, is autism on the rise? The authors suggest it could be due to increased awareness of the condition among parents and GPs, changing diagnostic criteria, or some other as yet unidentified environmental cause.